This invention relates to gauging the level of ink in an inkjet print cartridge by precisely determining the amount of ink that is ejected from a cartridge during printing.
An ink-jet printer typically includes one or more print cartridges that contain ink. In some designs, the cartridge has discrete reservoirs of more than one color of ink. Each reservoir is connected by a conduit to a printhead that is mounted to the body of the cartridge. The reservoir or supply of ink may be carried in the cartridge or remote from the cartridge. When a remote supply is used, the ink is delivered from the remote supply to the cartridge by a flexible tube to fill an intermediate reservoir adjacent to the printhead.
The cartridge is controlled for ejecting minute drops of ink from the printhead to a printing medium, such as paper, that is advanced through the printer. The ejection of the drops is controlled so that the drops form recognizable images on the paper. The cartridge is mounted to a carriage that scans across the medium as drops are ejected.
One can consider the portion of the print medium that is traversed by a printhead for receiving ink from the print head as a print swath. Between carriage scans, the paper is advanced so that the next swath of the image may be printed. Oftentimes, especially for color images, the carriage is scanned more than once across the same swath. With each such scan, a different combination of colors or droplet patterns may be printed until the printed swath is complete.
With thermal-type inkjet printers the printhead includes several resistors that are selectively driven (heated) with pulses of electrical current. The heat from each driven resistor is sufficient to form a vapor bubble in ink that fills an ink chamber that surrounds the resistor. The rapid expansion of the vapor bubble ejects or xe2x80x9cfiresxe2x80x9d an ink drop through a nozzle that is associated with the ink chamber. The chamber is refilled after each drop ejection with ink that flows into the chamber through a channel that connects with the conduit to the reservoir ink. Each printhead has numerous chambers and nozzles.
It is important to properly gauge the amount of ink remaining in a print cartridge. In this regard, it is best to replace a nearly empty cartridge with a full or nearly full one before a large (in terms of ink density) print operation is started. That is, print quality may suffer if a print cartridge is replaced during a printing task. Also, the printhead itself can fail and be damaged if it were operated (that is, driven with current pulses) after the supply of ink was depleted by an amount such that the ink chambers surrounding the resistors no longer filled. This printhead-damaging situation is characterized as xe2x80x9cdry firing.xe2x80x9d
Given the importance of accurately gauging ink levels, there have been provided in the past numerous attempts to monitor the amount of ink remaining in a supply or reservoir. For example, an optical sensor may be positioned near a transparent portion of an ink supply and configured to produce a signal when the light transmissive characteristics of that portion change in a manner that indicates the supply is nearing empty. The signal is converted to a human perceptible warning or notice (xe2x80x9cLow-on-Inkxe2x80x9d xe2x80x9cOut-of-Ink,xe2x80x9d etc) for indicating that the supply should be replaced.
However a low-on-ink or out-of-ink signal is produced, the printer is usually controlled so that a small amount of ink is reserved in the supply once an out-of-ink condition is reached. This reserve may be enough to enable the printer to complete printing of a sheet (rather than stopping during printing of a sheet) and xe2x80x9climp homexe2x80x9d to a service station carried in the printer. In any event, the reserve is large enough to ensure that no printhead dry firing occurs.
In one approach to gauging the amount of ink remaining in a supply or reservoir, the printer controller keeps track of the number of drops fired from the printhead and periodically updates a memory structure that initially reflects the amount of ink in a full cartridge. For example, a new cartridge would be characterized at the time of manufacture as having a given amount of ink, preferably measured in units of weight. A printer controller is provided (as by associated firmware) with this initial weight. As drops are fired, the printer controller accumulates the drop count and converts that count to a corresponding weight of expelled ink. This amount is subtracted from the initial weight of ink in the cartridge, and an appropriate warning signal is produced when the remaining weight is depleted by an amount indicting the printer is low on ink or out of ink.
The present invention generally follows the xe2x80x9cdrop countxe2x80x9d approach to ink level gauging and is directed to a method of making more precise the relationship between the expelled-drop count and the weight of ink actually expelled, thereby to provide more accurate ink level gauging.
By making the gauging more precise, the amount of reserve ink (which can be thought of as a safety factor) can be reduced, which leads to less wasted ink when a user replaces a cartridge. An attendant advantage to this is the production of more printed pages per cartridge.
The present invention may be used to supplement other ink level gauging approaches (such as the optical monitoring mentioned above), or as a stand-alone technique for precisely monitoring the level of ink in the cartridge.
The temperature of an operating printhead can vary considerably as a swath is printed. This variation in temperature is primarily due to the amount of ink that is printed (the print density) within the swath. Thus, when a portion of an image requires lots of ink, the printhead operating temperature will rise. As the printhead temperature increases, the weight of each expelled drop (that is, the xe2x80x9cdrop weightxe2x80x9d) also increases. Put another way, temperature changes from a normal or set point printhead operating temperature will cause changes in the drop weight that must be accounted for in gauging the amount of remaining ink. Generally, as the temperature increases, the drop weight increases.
As one aspect of the present invention, the printhead temperature is monitored as each swath of the image is printed. Moreover, temperature variations that occur within each swath are noted so that the corresponding intra-swath variations in drop weight are factored into the calculation of a xe2x80x9cnetxe2x80x9d ink drop weight that more closely approximates the drop weight actually ejected.
The method of the present invention also factors in the effect that printing frequency has on drop weight. The printing frequency is the rate with which inks drops are ejected and is measured in cycles, such as hertz (Hz). Generally, the ejected drop weight decreases as the printing frequency increases. As with temperature, printing frequency may vary considerably during printing of a swath. The present invention accounts for this intra-swath variation of printing frequency.
At some printing frequencies the effects of temperature changes on drop weight are much more pronounced than at other printing frequencies. Conversely, drop weights may not vary significantly with printing frequency changes within certain ranges of frequencies. Consequently, it is contemplated that the method of the present invention may, in some instances, account for only printhead temperature changes or only frequency changes. Normally, however, both temperature and frequency will be considered.
As another aspect of the present invention, the determination of ejected drop weight also accounts for drop weight variations that are attributable to normal use over the life of the printhead. That is, a printhead has a useful life that may be measured in tens of millions of ejected drops and, with other factors being equal, the average drop weight tends to increase during the life of the printhead. The drop weight variation over the life of the printhead is considered in the present invention.
As another aspect of the present invention, the method maintains information (preferably on the print cartridge) relating to the difference between the initial weight of a full cartridge and the net weight of the ink ejected from the cartridge. In other words, the ink level or amount of remaining ink is maintained in memory and made available for display to the user of the printer. This ink level is also adjusted from time to time to account for ink depletion resulting from evaporation.
The invention is primarily embodied in a printer control algorithm of a printing system that includes mechanisms (processor, temperature sensors, drop counters, display, etc) for efficiently performing the algorithm so that ink level data is continuously and precisely gauged and made available to the user.
Apparatus and methods for carrying out the invention are described in detail. Other advantages and features of the present invention will become clear upon review of the following portions of this specification and the drawings.